The present invention relates generally to passive restraint systems and, more particularly, to a gas generator which uses electrothermal energy to produce gas of sufficient quantity to inflate an air cushion of an occupant restraint system of the "air bag" type.
Occupant restraint systems of the "air bag" type have been developed in response to the need to better protect occupants of automobiles from serious injuries common in vehicular accidents. To be effective as a passive restraint , the air cushion of the system must be fully inflated within approximately 50 msec., or less, of crash impact which time depends upon, among other things, the time required to sense the magnitude and position of the crash.
Throughout the evolution of "air bag" type occupant restraint systems there has been the persistent problem of a lack of suitable inflation gas generating means. This is because the gas generator must be capable of generating enough gas, up to 500 liters, to completely inflate the air bag within the 20-50 msec. of available time. The system must produce generally non-toxic and non-combustible gas to inflate the air cushion because the air cushion ultimately vents into the passenger compartment on deflation and because of the possibility of a cushion failure in an actual crash situation or during an inadvertent inflation in a non-crash condition. Also, the gas generator must be capable of lying dormant under a wide range of environmental conditions for several years without affecting the operability of the system in the event of a crash.
The goal of rapid generation of non-toxic inflation gas having long-term operability has been met with varying levels of success.
The first systems attempting to meet the requirements of a truly functional "air bag" type passenger restraint systems used high pressure stored gas to inflate the air cushion. Upon sensing a deceleration greater than a predetermined threshold level, gas from the storage container would be released, inflating the air cushion. Although these devices adequately inflated the air cushion, they had numerous disadvantages including weight, size, cost, and reliability. An example of such a device is provided by U.S. Pat. No. 3,837,671, which is incorporated herein by reference.
Pyrotechnic gas generators have also been used, wherein a propellant, such as sodium azide, is burned to generate an adequate amount of gas upon vehicle impact. An example of gas generators which burn sodium azide is provided by U.S. Pat. No. 4,929,290, which is incorporated herein by reference. The sodium azide inflator has many drawbacks. First, the basic manufacture of the propellant is hazardous, there being a significant risk of accidental fire and explosion at least until the propellant is pelletized. Second, sodium azide, when ignited, can produce harmful by-products and partially combusted materials that can burn through the cushion material of the air cushion. To prevent the partially combusted materials from injuring the occupants, a gas filter is provided and the inside of the air cushion is specially coated to resist "burn through." Further shortcomings of sodium azide inflators include undesirable size, weight, cost and the fact that it is neither readily testable nor reusable.
A third gas generating device that has been proposed is a combination of the previously mentioned stored gas and combustible propellant devices. In this scheme, the propellant is ignited and the gas generated thereby is supplemented by the stored, high pressure gas. An example of such a device is provided by U.S. Pat. No. 3,966,226, which is incorporated herein by reference.
None of the aforementioned devices is particularly attractive due to the shortcomings mentioned. These limitations make the commercial production and implementation of "air bag" type occupant restraint systems difficult. As consumer demand and governmental safety requirements continue to increase, there exists a need for a gas generating device which overcomes the limitations of those noted above.